The present invention relates to an objective lens used for an optical pick-up apparatus and an optical pick-up apparatus, and particularly relates to an objective lens for conducting recording/reproducing of optical information recording media whose recording density is different, by one objective lens, and an optical pick-up apparatus.
In the present, many kinds of optical information recording media exist and the standard of these optical information recording media is decided as shown in [Table 1]. In this connection, hereinafter (including the lens data in the table), an exponent of 10 (for example, 2.5xc3x9710xe2x88x923) is expressed by E (for example, 2.5xc3x97Exe2x88x923).
Herein, as an example in which the interchangeability of mutual optical information recording media whose recording density is different is required, there is DVD and CD. In these optical information recording media, as shown in [Table 1], the transparent substrate thickness are respectively different. In order to secure the interchangeability, it is necessary that the spherical aberration generated by the difference of this transparent substrate thickness is corrected by any means. Further, in the DVD and CD, because the required numerical aperture is different, any countermeasure is necessary also for this.
In order to realize an optical pick-up apparatus having the interchangeability of the DVD/CD, an objective lens in which a diffractive structure is provided, is developed. As such an objective lens, there is an objective lens in which for example, on one side surface of the objective lens, the diffractive structure which is different in the inside and outside of a specific distance h from the optical axis is provided, and in the inside area, the spherical aberration is corrected for the respective transparent substrate thickness and in the outside area, the spherical aberration is corrected only for the DVD, and for the CD, the spherical aberration is not corrected, but flared. When the objective lens is structured in this manner, on each optical information recording medium, the light converging spot which is required at the time of record or reproduction of each information, can be adequately formed.
In this connection, by using such a diffractive structure for the objective lens, the correction of the spherical aberration when the recording or reproducing is carried out for both DVD/CD, can be carried out comparatively easily. However, as an optical characteristic deterioration factor by which the recording or reproducing of the adequate information is prevented, there is also a coma other than the spherical aberration. When the coma is large, by the inclination of the objective lens generated due to the assembling error, when the light flux inclined to the optical axis is incident, there is a possibility to prevent that the adequate spot is formed on the optical information recording medium. However, when the above-described diffractive structure is used, the spherical aberration at the time of use of both DVD/CD can be reduced together, but for the coma, there is a problem that both can not be simultaneously corrected.
As the light source of the optical pick-up apparatus specifically to attain the interchangeability of the DVD/CD, the light source in which 2 semiconductor lasers which are called so-called 2 laser 1 package are attached to one substrate and formed as one unit, is well known. By using such a light source, when the information is going to be recorded or reproduced onto both of DVD/CD, for example, when the light source for the DVD is arranged on the optical axis of the objective lens, the light source for the CD is arranged at the position shifted from the optical axis without fail, accordingly, in this case, it can be said to be preferable that the coma is extremely reduced at the time of the use of CD.
The present invention is attained in view of the above problems, and the object of the present invention is to provide the objective lens for the optical pick-up apparatus and the optical pick-up apparatus by which the coma can be corrected with the good balance, and the recording or reproducing of the information can be adequately carried out onto different optical information recording media.